1. Field of the Invention
This invention concerns a method for recovering petroleum from viscous, asphaltic petroleum containing formations such as tar sand deposits utilizing a combined in situ solvent deasphalting process with in situ combustion.
2. Description of the Prior Art of the Prior Art
There are known to exist in many locations throughout the world, large deposits of viscous, asphaltic petroleum containing formation. The most famous such formations are the tar sands, also known as oil sands, or bituminous sands. The largest and most famous deposit is located in the Athabasca area in the Northeastern part of the Providence of Alberta, Canada, which has been estimated in the literature to contain over 700 billion barrels of petroleum. Other large tar sand deposits are found in the United States and Venezuela, and smaller deposits are located in Europe and Asia.
Tar sand deposits differ from conventional petroleum reservoirs in a number of respects. The hydrocarbon portion is highly bituminous in character, and is much more viscous than conventional petroleum. The deposits contain sand, predominately fine quartz sand, which is covered with a film of water. Surrounding the wetted sand grains and essentially filling the void volume among them is a film of bituminous hydrocarbon. The balance of the void volume may be filled with connate water, and sometimes small quantities of gas are encountered, which is usually air or methane. The sand grain represent about 65% by volume of the total volume of the deposit, which is equivalent to about 85% by weight. The sum of the bitumen and water concentration will generally equal about 17%, with the bitumen portion thereof varying from about 3% to about 15% by weight. The most unique difference between the tar sand deposit and conventional petroleum reservoirs is the fact that in the best developed tar sands intervals the bitumen is in fact the continuous phase of the reservoir, and the sand grains are suspended in the bitumen.
The bitumen properties vary throughout the world, although they are fairly constant in broad geographic areas. For example, the bitumen characteristics of the United States tar sand deposits are fairly consistently the same. The density of the bitumen is generally slightly greater than the density of water at 60.degree. F. About 50% of the bitumen is distillable without cracking. The sulfur content of the bitumen varies from 4.5 to 5%. Thus it can be appreciated that considerable on site processing must be undertaken in order to transport and utilize the hydrocarbons contained from tar sand deposits.
There are two basic approaches for recovering the hydrocarbon or bituminous material from tar sand deposits. The tar sands may be mined and transported to a processing plant where the bitumen is extracted and the sand is discharged, or the separation of bitumen from sand may be accomplished within the reservoir by an in situ process. In situ recovery processes are closely related to so-called supplemental recovery or secondary recovery of crude oils from conventional oil reservoirs, although there are differences imposed by the unique characteristics of tar sand deposits.
The in situ processes for recovering bitumen from tar sand deposits may generally be classified as:
1. Thermal methods, including fire flooding also known as in situ combustion, and steam injection, and
2. Emulsion steam processes in which an emulsifier is injected into the formation via a previously created fluid communication path, and steam is injected to partially liquify the bitumen and to form an oil in water emulsion in order to create a fluid having suitable flow properties for recovery from the subterranean deposit.
In in situ combustion or fire flooding, heat for viscosity reduction is generated within the formation by injecting air into the formation and igniting the formations so as to produce a combustion reaction within the tar sand deposit itself. Some of the hydrocarbons are consumed in the in situ combustion reaction, but a substantial proportion of the hydrocarbons are recovered.
In U.S. Pat. No. 3,062,282, A. R. Schlercher, Nov. 6, 1962, there is disclosed an in situ combustion type of oil recovery process wherein a flushing fluid such as air or normally gaseous, paraffinic hydrocarbons such as propane, is injected into a formation preheated to a temperature of at least 300.degree. F., to displace heavy liquid hydrocarbons therefrom, followed by in situ combustion.
If it is desired to apply in situ combustion techniques to formations containing very viscous hydro-specifically tar sand deposits, it is often impossible to raise the reaction temperature high enough to achieve the desired mobility of the fluid by thermal means alone. It is recognized in the prior art that come in situ thermal cracking of the hydrocarbon materials may be accomplished if the temperatures can be raised to a sufficiently high level. In conventional oil reservoirs it is possible to operate under conditions so as to achieve a limited degree of thermal cracking by increasing the air injection pressure so as to increase the temperature of the combustion reaction. This is usually not possible in tar sand deposits, since a great many of these deposits are generally relatively shallow, e.g. in order of 100 to 1,000 feet deep as compared to the more conventional hydrocarbon deposits. If the air injection pressure is raised in shallow deposits, fracturing of the formation results, which is detrimental to oil recovery by in situ combustion. Furthermore, a higher degree of thermal cracking must be accomplished in tar sand deposits in order to achieve a fluid mobility sufficient to permits recovery.
Thus it can be seen that there is a substantial unfulfilled need for a method for performing in situ combustion in tar sand deposits in such a way that a relatively high combustion temperature is obtained, so that appreciable thermal cracking of the bituminous hydrocarbon materials occurs.
Most tar sand deposits contain hydrocarbon materials which are a mixture of high molecular weight bituminous materials and more conventional lower molecular petroleum components. Since the bituminous materials require surface treating to render them sufficiently mobile for pipeline transportation to remotely located refineries, the more mobile conventional petroleum materials have a higher dollar value. Accordingly, if any of the hydrocarbon material present in tar sands must be consumed by the in situ combustion reaction, it would be highly desirable to utilize a larger percentage of less valuable, less mobile bituminous materials as the fuel for the in situa combustion reaction. Accordingly, there is also a substantial need for a method for conducting in situ combustion oil recovery under conditions which preferentially consume the high molecular weight bituminous materials as fuel for the in situ combustion reaction.